Men are more likely than women to slow in the marathon

Studies on nonelite distance runners suggest that men are more likely than women to slow their pace in a marathon.

PURPOSE

This study determined the reliability of the sex difference in pacing across many marathons and after adjusting women's performances by 12% to address men's greater maximal oxygen uptake and also incorporating information on racing experience.

METHODS

Data were acquired from 14 US marathons in 2011 and encompassed 91,929 performances. For 2929 runners, we obtained experience data from a race-aggregating Web site. We operationalized pace maintenance as the percentage change in pace observed in the second half of the marathon relative to the first half. Pace maintenance was analyzed as a continuous variable and as two categorical variables, as follows: "maintain the pace," defined as slowing <10%, and "marked slowing," defined as slowing ≥30%.

RESULTS

The mean change in pace was 15.6% and 11.7% for men and women, respectively (P < 0.0001). This sex difference was significant for all 14 marathons. The odds for women were 1.46 (95% confidence interval, 1.41-1.50; P < 0.0001) times higher than men to maintain the pace and 0.36 (95% confidence interval, 0.34-0.38; P < 0.0001) times that of men to exhibit marked slowing. Slower finishing times were associated with greater slowing, especially in men (interaction, P < 0.0001). However, the sex difference in pacing occurred across age and finishing time groups. Making the 12% adjustment to women's performances lessened the magnitude of the sex difference in pacing but not its occurrence. Although greater experience was associated with less slowing, controlling for the experience variables did not eliminate the sex difference in pacing.

CONCLUSIONS

The sex difference in pacing is robust. It may reflect sex differences in physiology, decision making, or both.

Sex Differences in Arm Muscle Fatigability With Cognitive Demand in Older Adults

BACKGROUND

Muscle fatigability can increase when a stressful, cognitively demanding task is imposed during a low-force fatiguing contraction with the arm muscles, especially in women. Whether this occurs among older adults (>60 years) is currently unknown.

QUESTIONS/PURPOSES

We aimed to determine if higher cognitive demands, stratified by sex, increased fatigability in older adults (>60 years). Secondarily, we assessed if varying cognitive demand resulted in decreased steadiness and was explained by anxiety or cortisol levels.

METHODS

Seventeen older women (70±6 years) and 13 older men (71±5 years) performed a sustained, isometric, fatiguing contraction at 20% of maximal voluntary contraction until task failure during three sessions: high cognitive demand (high CD=mental subtraction by 13); low cognitive demand (low CD=mental subtraction by 1); and control (no subtraction).

RESULTS

Fatigability was greater when high and low CD were performed during the fatiguing contraction for the women but not for the men. In women, time to failure with high CD was 16±8 minutes and with low CD was 17±4 minutes, both of which were shorter than time to failure in control contractions (21±7 minutes; high CD mean difference: 5 minutes [95% confidence interval {CI}, 0.78-9.89], p=0.02; low CD mean difference: 4 minutes [95% CI, 0.57-7.31], p=0.03). However, in men, no differences were detected in time to failure with cognitive demand (control: 13±5 minutes; high CD mean difference: -0.09 minutes [95% CI, -2.8 to 2.7], p=1.00; low CD mean difference: 0.75 minutes [95% CI, -1.1 to 2.6], p=0.85). Steadiness decreased (force fluctuations increased) more during high CD than control. Elevated anxiety, mean arterial pressure, and salivary cortisol levels in both men and women did not explain the greater fatigability during high CD.

CONCLUSIONS

Older women but not men showed marked increases in fatigability when low or high CD was imposed during sustained static contractions with the elbow flexor muscles and contrasts with previous findings for the lower limb. Steadiness decreased in both sexes when high CD was imposed.

CLINICAL RELEVANCE

Older women are susceptible to greater fatigability of the upper limb with heightened mental activity during sustained postural contractions, which are the foundation of many work-related tasks.

Effects of sex and gender on adaptation to space: musculoskeletal health

There is considerable variability among individuals in musculoskeletal response to long-duration spaceflight. The specific origin of the individual variability is unknown but is almost certainly influenced by the details of other mission conditions such as individual differences in exercise countermeasures, particularly intensity of exercise, dietary intake, medication use, stress, sleep, psychological profiles, and actual mission task demands. In addition to variations in mission conditions, genetic differences may account for some aspect of individual variability. Generally, this individual variability exceeds the variability between sexes that adds to the complexity of understanding sex differences alone. Research specifically related to sex differences of the musculoskeletal system during unloading is presented and discussed.

Sex differences with aging in the fatigability of dynamic contractions

This study determined the sex difference with aging in fatigability of the elbow flexor muscles during a dynamic fatiguing task, and explored the associated mechanisms. We compared fatigability of the elbow flexor muscles in 18 young (20.2 ± 1 years: 9 men) and 36 old adults (73.5 ± 1 years: 16 men) during and in recovery from repeated dynamic contractions (~60°/s) with a load equivalent to 20% of maximal voluntary isometric contraction (MVIC) torque until failure. Transcranial magnetic stimulation (TMS) was used to assess supraspinal fatigue (an increase in the superimposed twitch, SIT) and the peak rate of muscle relaxation. Time to failure was briefer for the men than the women (6.1 ± 2.1 vs. 9.7 ± 5.5 min, respectively; P=0.02) with no difference between young and old adults (7.2 ± 2.9 vs. 8.4 ± 5.2 min, respectively, P=0.45) and no interaction (P>0.05). The relative decline in peak relaxation rate with fatigability was similar for young and old adults (P=0.11), but greater for men than women (P=0.046). Supraspinal fatigue increased for all groups and was associated with the time to failure (P<0.05). Regression analysis however, indicated that the time to failure was best predicted by the peak relaxation rate (baseline values and slowing with fatigability) (r(2)=0.55). Rate-limiting contractile mechanisms (e.g. excitation-contraction coupling) were responsible for the increased fatigability of the elbow flexors of men compared with women for a dynamic fatiguing task of slow angular velocity, and this sex difference was maintained with aging. The age difference in fatigability for the dynamic task was diminished for both sexes relative to what is typically observed with isometric fatiguing contractions.

Stroke-related effects on maximal dynamic hip flexor fatigability and functional implications

INTRODUCTION

Stroke-related changes in maximal dynamic hip flexor muscle fatigability may be more relevant functionally than isometric hip flexor fatigability.

METHODS

Ten chronic stroke survivors performed 5 sets of 30 hip flexion maximal dynamic voluntary contractions (MDVC). A maximal isometric voluntary contraction (MIVC) was performed before and after completion of the dynamic contractions. Both the paretic and nonparetic legs were tested.

RESULTS

Reduction in hip flexion MDVC torque in the paretic leg (44.7%) was larger than the nonparetic leg (31.7%). The paretic leg had a larger reduction in rectus femoris EMG (28.9%) between the first and last set of MDVCs than the nonparetic leg (7.4%). Reduction in paretic leg MDVC torque was correlated with self-selected walking speed (r2=0.43), while reduction in MIVC torque was not (r2=0.11).

CONCLUSIONS

Reductions in maximal dynamic torque of paretic hip flexors may be a better predictor of walking function than reductions in maximal isometric contractions.

Age and sex differences in steadiness of elbow flexor muscles with imposed cognitive demand

PURPOSE

These studies determined (1) age- and sex-related differences in steadiness of isometric contractions when high cognitive demand was imposed across a range of forces with the elbow flexor muscles (study 1) and; (2) sex differences in steadiness among older adults when low cognitive demand was imposed (study 2).

METHODS

36 young adults (18-25 years; 18 women) and 30 older adults (60-82 years; 17 women) performed isometric contractions at 5, 30 and 40 % of maximum voluntary contraction (MVC). Study 1 involved a high-cognitive demand session (serial subtractions by 13 during the contraction) and a control session (no mental math). Study 2 (older adults only) involved a low-cognitive demand session (subtracting by 1s).

RESULTS

Older individuals exhibited greater increases in force fluctuations (coefficient of variation of force, CV) with high cognitive demand than young adults, with the largest age difference at 5 % MVC (P = 0.01). Older adults had greater agonist EMG activity with high-cognitive demand and women had greater coactivation than men (P < 0.05). In study 2, CV of force increased with low cognitive demand for the older women but not for the older men (P = 0.03).

CONCLUSION

Older adults had reduced steadiness and increased muscle activation when high cognitive demand was imposed while low cognitive demand induced increased force fluctuations in older women but not older men. These findings have implications for daily and work-related tasks that involve cognitive demand performed simultaneously during submaximal isometric contractions in an aging workforce.

Muscle fatigability and control of force in men with posttraumatic stress disorder

INTRODUCTION

Acute stress can increase fatigability and decrease steadiness of sustained low-force contractions that are required for functional tasks in upper limb muscles. Whether motor performance is more impaired in people with a chronic stress disorder is not known.

PURPOSE

This study compared the fatigability and steadiness (force fluctuations) of handgrip muscles in veterans with posttraumatic stress disorder (PTSD) and civilian controls in the presence and absence of varying levels of cognitive demand.

METHODS

Eighteen veterans with PTSD and 21 healthy controls (33 ± 9 yr) attended three randomized experimental sessions to perform an isometric fatiguing contraction (20% of maximal strength) with the handgrip muscles. Two sessions involved performing a cognitive task during the fatiguing contraction: 1) difficult mental math task (stressor) and 2) a simple mental math task (mental attentiveness). A third session involved a fatiguing contraction with no mental task (control).

RESULTS

Stress elevated heart rate, blood pressure, and levels of anxiety in veterans with PTSD (P < 0.05) but blunted cortisol levels (P < 0.05). Time to failure was briefer (7.2 ± 2.5 vs 9.3 ± 5.2 min, P = 0.03), and force fluctuations increased at a greater rate for veterans with PTSD than for controls (P < 0.05). Cognitive stress did not influence time to failure or force fluctuations for either group (P > 0.05).

CONCLUSIONS

Veterans with PTSD demonstrated greater fatigability and loss of steadiness (greater force fluctuations) of the handgrip muscles compared with healthy controls.

SIGNIFICANCE

Male veterans with PTSD demonstrated altered neuromuscular function of arm muscles that potentially affects functional tasks during daily, ergonomic, and military activities.

Brain areas associated with force steadiness and intensity during isometric ankle dorsiflexion in men and women

Although maintenance of steady contractions is required for many daily tasks, there is little understanding of brain areas that modulate lower limb force accuracy. Functional magnetic resonance imaging was used to determine brain areas associated with steadiness and force during static (isometric) lower limb target-matching contractions at low and high intensities. Fourteen young adults (6 men and 8 women; 27.1 ± 9.1 years) performed three sets of 16-s isometric contractions with the ankle dorsiflexor muscles at 10, 30, 50, and 70 % of maximal voluntary contraction (MVC). Percent signal changes (PSCs, %) of the blood oxygenation level-dependent response were extracted for each contraction using region of interest analysis. Mean PSC increased with contraction intensity in the contralateral primary motor area (M1), supplementary motor area, putamen, pallidum cingulate cortex, and ipsilateral cerebellum (p < 0.05). The amplitude of force fluctuations (standard deviation, SD) increased from 10 to 70 % MVC but relative to the mean force (coefficient of variation, CV %) was greatest at 10 % MVC. The CV of force was associated with PSC in the ipsilateral parietal lobule (r = -0.28), putamen (r = -0.29), insula (r = -0.33), and contralateral superior frontal gyrus (r = -0.33, p < 0.05). There were minimal sex differences in brain activation across the isometric motor tasks indicating men and women were similarly motivated and able to activate cortical motor centers during static tasks. Control of steady lower limb contractions involves cortical and subcortical motor areas in both men and women and provides insight into key areas for potential cortical plasticity with impaired or enhanced leg function.

Functional implications of impaired control of submaximal hip flexion following stroke

INTRODUCTION

We quantified submaximal torque regulation during low to moderate intensity isometric hip flexion contractions in individuals with stroke and the associations with leg function.

METHODS

Ten participants with chronic stroke and 10 controls performed isometric hip flexion contractions at 5%, 10%, 15%, 20%, and 40% of maximal voluntary contraction (MVC) in paretic, nonparetic, and control legs.

RESULTS

Participants with stroke had larger torque fluctuations (coefficient of variation, CV), for both the paretic and nonparetic legs, than controls (P < 0.05) with the largest CV at 5% MVC in the paretic leg (P < 0.05). The paretic CV correlated with walking speed (r2 = 0.54) and Berg Balance Score (r2 = 0.40). At 5% MVC, there were larger torque fluctuations in the contralateral leg during paretic contractions compared with the control leg.

CONCLUSIONS

Impaired low-force regulation of paretic leg hip flexion can be functionally relevant and related to control versus strength deficits poststroke.

Motor Variability during Sustained Contractions Increases with Cognitive Demand in Older Adults

To expose cortical involvement in age-related changes in motor performance, we compared steadiness (force fluctuations) and fatigability of submaximal isometric contractions with the ankle dorsiflexor muscles in older and young adults and with varying levels of cognitive demand imposed. Sixteen young (20.4 ± 2.1 year: 8 men, 9 women) and 17 older adults (68.8 ± 4.4 years: 9 men, 8 women) attended three sessions and performed a 40 s isometric contraction at 5% maximal voluntary contraction (MVC) force followed by an isometric contraction at 30% MVC until task failure. The cognitive demand required during the submaximal contractions in each session differed as follows: (1) high-cognitive demand session where difficult mental math was imposed (counting backward by 13 from a 4-digit number); (2) low-cognitive demand session which involved simple mental math (counting backward by 1); and (3) control session with no mental math. Anxiety was elevated during the high-cognitive demand session compared with other sessions for both age groups but more so for the older adults than young adults (p  < 0.05). Older adults had larger force fluctuations than young adults during: (1) the 5% MVC task as cognitive demand increased (p  = 0.007), and (2) the fatiguing contraction for all sessions (p  = 0.002). Time to task failure did not differ between sessions or age groups (p  > 0.05), but the variability between sessions (standard deviation of three sessions) was greater for older adults than young (2.02 ± 1.05 vs. 1.25 ± 0.51 min, p  < 0.05). Thus, variability in lower limb motor performance for low- and moderate-force isometric tasks increased with age and was exacerbated when cognitive demand was imposed, and may be related to modulation of synergist and antagonist muscles and an altered neural strategy with age originating from central sources. These data have significant implications for cognitively demanding low-force motor tasks that are relevant to functional and ergonomic in an aging workforce.

The stroke-related effects of hip flexion fatigue on over ground walking

Individuals post stroke often rely more on hip flexors for limb advancement during walking due to distal weakness but the effects of muscle fatigue in this group is not known. The purpose of this study was to quantify how stroke affects the influence of hip flexor fatigue on over ground walking kinematics and performance and muscle activation. Ten individuals with chronic stroke and 10 without stroke (controls) participated in the study. Maximal walking speed, walking distance, muscle electromyograms (EMG), and lower extremity joint kinematics were compared before and after dynamic, submaximal fatiguing contractions of the hip flexors (30% maximal load) performed until failure of the task. Task duration and decline in hip flexion maximal voluntary contraction (MVC) and power were used to assess fatigue. The stroke and control groups had similar task durations and percent reductions in MVC force following fatiguing contractions. Compared with controls, individuals with stroke had larger percent reductions in maximal walking speed, greater decrements in hip range of motion and peak velocity during swing, greater decrements in ankle velocity and lack of modulation of hip flexor EMG following fatiguing dynamic hip flexion contractions. For a given level of fatigue, the impact on walking function was more profound in individuals with stroke than neurologically intact individuals, and a decreased ability to up regulate hip flexor muscle activity may contribute. These data highlight the importance of monitoring the effect of hip flexor muscle activity during exercise or performance of activities of daily living on walking function post stroke.

Stressor-induced increase in muscle fatigability of young men and women is predicted by strength but not voluntary activation

This study investigated mechanisms for the stressor-induced changes in muscle fatigability in men and women. Participants performed an isometric-fatiguing contraction at 20% maximal voluntary contraction (MVC) until failure with the elbow flexor muscles. Study one (n = 55; 29 women) involved two experimental sessions: 1) a high-stressor session that required a difficult mental-math task before and during a fatiguing contraction and 2) a control session with no mental math. For some participants (n = 28; 14 women), cortical stimulation was used to examine mechanisms that contributed to muscle fatigability during the high-stressor and control sessions. Study two (n = 23; nine women) determined the influence of a low stressor, i.e., a simple mental-math task, on muscle fatigability. In study one, the time-to-task failure was less for the high-stressor session than control (P < 0.05) for women (19.4%) and men (9.5%): the sex difference response disappeared when covaried for initial strength (MVC). MVC force, voluntary activation, and peak-twitch amplitude decreased similarly for the control and high-stressor sessions (P < 0.05). In study two, the time-to-task failure of men or women was not influenced by the low stressor (P > 0.05). The greater fatigability, when exposed to a high stressor during a low-force task, was not exclusive to women but involved a strength-related mechanism in both weaker men and women that accelerated declines in voluntary activation and slowing of contractile properties.

Effects of type 1 diabetes, sprint training and sex on skeletal muscle sarcoplasmic reticulum Ca2+ uptake and Ca2+-ATPase activity

Calcium cycling is integral to muscle performance during the rapid muscle contraction and relaxation of high-intensity exercise. Ca(2+) handling is altered by diabetes mellitus, but has not previously been investigated in human skeletal muscle. We investigated effects of high-intensity exercise and sprint training on skeletal muscle Ca(2+) regulation among men and women with type 1 diabetes (T1D, n = 8, 3F, 5M) and matched non-diabetic controls (CON, n = 8, 3F, 5M). Secondarily, we examined sex differences in Ca(2+) regulation. Subjects undertook 7 weeks of three times-weekly cycle sprint training. Before and after training, performance was measured, and blood and muscle were sampled at rest and after high-intensity exercise. In T1D, higher Ca(2+)-ATPase activity (+28%) and Ca(2+) uptake (+21%) than in CON were evident across both times and days (P < 0.05), but performance was similar. In T1D, resting Ca(2+)-ATPase activity correlated with work performed until exhaustion (r = 0.7, P < 0.01). Ca(2+)-ATPase activity, but not Ca(2+) uptake, was lower (-24%, P < 0.05) among the women across both times and days. Intense exercise did not alter Ca(2+)-ATPase activity in T1D or CON. However, sex differences were evident: Ca(2+)-ATPase was reduced with exercise among men but increased among women across both days (time × sex interaction, P < 0.05). Sprint training reduced Ca(2+)-ATPase (-8%, P < 0.05), but not Ca(2+) uptake, in T1D and CON. In summary, skeletal muscle Ca(2+) resequestration capacity was increased in T1D, but performance was not greater than CON. Sprint training reduced Ca(2+)-ATPase in T1D and CON. Sex differences in Ca(2+)-ATPase activity were evident and may be linked with fibre type proportion differences.